Quick update:
I am testing the tachometer using interrupts to record the pedaling frequency (right now simulated by button presses).
I salvaged two LiIon cells from an old PDA, powering Arduino with 7.2V!
Also, I ordered MAX6955 and 6956 ICs, now driving a row of LEDs and one 7 segment display over I2C! ;D And they include five general purpose I/Os which I can use for buttons in the handlebars. Perfect!
Small test video here.
Next step: Order some more 7 segment elements, attach the control buttons, write the UI.